ZnO has been used for quite a while as green fluorescent material for use in fluorescent display tubes or electrophotographic photosensitive materials and has also been applied as a constituent material in surface elastic wave devices. In recent years, a ZnO film has been noted as being an inexpensive transparent electroconductive film. Various studies have been made for its application to functional devices such as thin film solar cells, liquid crystal displays and electroluminescence (EL) devices. As for the method of forming such ZnO thin film, there have been known magnetron sputtering method, RF sputtering method, CVD method, spray method, etc. For example, the magnetron sputtering method is disclosed in Journal of Crystal Growth, 47 (1979), p 171; the RF sputtering method is disclosed in Japanese Journal of Applied Physics, 22 (1983), p, L 245; and the CVD method and the spray method are disclosed in Current Topics In Materials Science, 7 (1981), p 148 respectively.
Incidentally, in the application to thin film solar cells, liquid crystal display devices, EL device, etc., it is important that the ZnO thin film as formed has a high light permeability and a low volumic resistivity. Particularly, its volumic resistivity has a close relation to the structure of the film and it greatly depends on the formation method. That is, for reducing the volumic resistivity, it is necessary that the ZnO thin film be in the form of a crystalline thin film oriented along C-axis in the direction perpendicular to the plane of the film. The crystallinity greatly depends on the film-forming method and conditions. Further, in view of the domestic application use, it is necessary that under the method as employed, a ZnO thin film of good crystallinity can be formed easily on an inexpensive substrate, for example, a glass substrate at a good mass-productivity for providing inexpensive devices.
However, practical applicable ZnO thin films have not yet been provided by any of the above-mentioned methods because of the drawbacks as described below. That is, as shown in Japanese Journal of Applied Physics, 21 (1982), p 688, there has been a problem in the RF sputtering method including the magnetron sputtering method that high speed neutral oxygen atoms or oxygen negative ions caused in the film-forming space give impact shocks to the film surface to disturb the C-axis orientation, thereby increasing the resistivity upon forming the film.
In the CVD process, although the controllability is good and the mass-productivity is satisfactory, since a high temperature in excess of 500.degree. C. is required because of the utilization of hydrolysis of chlorides, and thermal decomposition of organic compounds, there is restriction to the substrate used. Further, the spray method is suitable to the production of low cost films since the apparatus is inexpensive and the steps are simple, but the controllability is poor and the reproducibility of the film property is not satisfactory.
If a ZnO thin film of high light permeability and low resistivity can be formed on an inexpensive substrate or on a functional film deposited to the surface of such a substrate, the final module cost of the device can be reduced and, particularly, inexpensive photovoltaic devices which can be served for practical power source in domestic equipments or power appliance in thin film solar cells can be provide to the market. However, this can not be expected be attain by the conventional technique as described above accordingly, there has been an urgent demand for establishing a method of forming a thin film capable of depositing a ZnO thin film of good quality at a low temperature on a desired substrate.